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Abstract

Using a pulse-chase 1321 CO2 plant labeling experiment we compared the flow of plant
22 carbon into macromolecular fractions of root-associated soil microorganisms. Time
dependent 1323 C dilution patterns in microbial cellular fractions were used to calculate their
24 turnover time. The turnover times of microbial biomolecules were found to vary: microbial
25 RNA (19 h) and DNA (30 h) turned over fastest followed by chloroform fumigation
26 extraction-derived soluble cell lysis products (14 d), while phospholipid fatty acids (PLFAs)
had the slowest turnover (42 d). PLFA/NLFA 1327 C analyses suggest that both mutualistic
28 arbuscular mycorrhizal and saprophytic fungi are dominant in initial plant carbon uptake. In
contrast, high initial 1329 C enrichment in RNA hints at bacterial importance in initial C uptake
30 due to the dominance of bacterial derived RNA in total extracts of soil RNA. To explain this
31 discrepancy, we observed low renewal rate of bacterial lipids, which may therefore bias lipid
32 fatty acid based interpretations of the role of bacteria in soil microbial food webs. Based on
33 our findings, we question current assumptions regarding plant-microbe carbon flux and
34 suggest that the rhizosphere bacterial contribution to plant assimilate uptake could be higher.
35 This highlights the need for more detailed quantitative investigations with nucleic acid 36 biomarkers to further validate these findings.